1. Technical Field:
The invention is related to plasma reactors for processing semiconductor integrated circuit wafers and specifically to improvements in the gas injection and distribution apparatus employed in such reactors.
2. Background Art:
A plasma reactor for processing semiconductor integrated circuit wafers, such as a metal etch reactor, typically includes a vacuum chamber, a pedestal for supporting the wafer in the chamber, a plasma RF power source and a gas injection source for supplying gases through the top of the chamber and a gas distribution plate near the chamber ceiling for distributing the incoming gases in a uniform manner. If the reactor is an inductively coupled reactor, then it can include a coil antenna around the chamber connected to the plasma RF power source, and the wafer pedestal can be connected to a bias RF power source. In other types of plasma reactors (such, for example, a reactive ion etch reactor), there is no coil antenna and the plasma RF power source is connected to the wafer pedestal. For plasma etch processes, the incoming gas includes an etchant species such as chlorine and/or boron tri-chloride, for example. The gas distribution plate is typically a flat plate covering the reactor chamber ceiling about 100-150 mils thick with about one hundred holes or orifices therethrough, each hole being no more than about 20-30 mils in diameter in order to prevent penetration of the plasma into the holes. The backside or top surface of the gas distribution plate is coupled to a gas injection source or port which extends through the chamber top or lid while the front side faces downwardly toward the chamber interior and the wafer.
The gas distribution plate must be fairly large to provide a reasonably uniform gas distribution within the chamber and to provide a means of controlling the electrical potential at the top of the reactor chamber. Typically, the gas distribution plate is a ground plane. For this purpose, the gas distribution plate must be made of a conductive material such as aluminum.
The basic problem with such a gas distribution plate is that it is subject to corrosion from the gases introduced through it into the reactor chamber. The gas distribution plate front surface is directly exposed to the plasma within the chamber and is therefore susceptible to attack therefrom. The gas distribution plate back surface is coupled to the gas supply inlet and the gases contacting the back surface are at a relatively high pressure due to the back pressure caused by the small size of the orifices or holes through the gas distribution plate. This high back pressure increases the rate at which the gas distribution plate back surface is corroded. In order to minimize such corrosion, the gas distribution plate is anodized to provide an aluminum oxide film on its surface. Unfortunately, in etch reactors employing mixtures of chlorine and boron tri-chloride gases, the boron tri-chloride etches the anodized surface (the aluminum oxide film) while the chlorine etches the aluminum underneath to undermine the aluminum oxide film.
Such problems are particularly acute at the orifices or holes through the gas distribution plate. In order to prevent a plasma glow from forming in the holes and behind the plate, the diameters of these holes cannot exceed the plasma sheath thickness, which is approximately 20-30 mils. Such small holes must be either drilled mechanically or by a laser, either of which forms a hole with sharp edges and roughens the interior surface of each hole. The sharp edges around each hole and the rough interior surfaces of each hole are eroded by the flow of chlorine and tri-chloride gases. If the gas distribution plate is anodized, cracks in the anodization are difficult to avoid at the sharp edges of the holes. At these edges, the anodization is susceptible to erosion due to gas flow and corrosion due to penetration of etchant gases into the anodization cracks and etching of the underlying aluminum. All such erosion and corrosion creates particle contamination which can lead to fatal defects in the integrated circuits on the wafer being processed in the chamber. In an effort to reduce such problems, the gas distribution plate hole edges may be routed and the hole interior surfaces and edges may be honed with a diamond powder. However, such efforts merely reduce the magnitude of the problem but do not solve the problem.
In view of the foregoing problems, one limitation of such plasma reactors has been that the gas distribution plate typically can only be used for two complete process cycles of the reactor, a new gas distribution plate being required thereafter. (The term "process cycle" as used herein refers to the time between successive chamber cleaning operations.) This avoids many of the foregoing problems but greatly increases the unproductive "down" time and capital costs or expenditures incurred while operating such a plasma reactor. Furthermore, enhanced corrosion of the top or lid behind (or above) the gas distribution plate is often observed.
Another problem with such reactors, particularly etch reactors, is that the etch rate near the wafer periphery is much greater than the etch rate near the wafer center. This is due at least in part to the fact that the etchant species are being consumed at a maximum rate near the wafer and may therefore be scarce in the region near the wafer center, while little or no consumption of etchant species occurs beyond the edge of the wafer so that a plentiful supply of etchant species exists just beyond the wafer periphery capable of sustaining very high etch rates near the wafer periphery. The etch rate across the wafer diameter may be rendered less non-uniform by providing for a greater gas flow through the gas distribution plate over the wafer center and a lesser gas flow over the wafer periphery. This is accomplished by providing more orifices or holes per unit area in the gas distribution plate over the wafer center and less over the wafer periphery. Etch rate uniformity across the wafer diameter to within 5 percent has been achieved, and is satisfactory.
One technique for further enhancing the uniformity of the etch rate across the wafer diameter is to provide a so-called focus ring, which may be an annular vertical high wall up to several centimeters in height surrounding the wafer periphery. This wall or focus ring stops or retards the replenishment of etchant species at the wafer periphery.